1. Field of the Invention
The present invention relates to switching power supplies and integrated circuits for controlling such power supplies, and more particularly to a circuit for controlling the minimum operating voltage of the integrated control circuit of a switching power supply.
2. Description of Related Art
A common necessity to all switching power supplies is to stop its operation when the supply voltage of the integrated control circuit of the power supply is too low.
In fact, the control and, above all, the driving of the power switching element (almost always an N channel enrichment MOSFET) requires that the voltage that supplies the integrated control circuit is higher than a minimum value so that not only the internal circuits of the integrated device are adequately supplied but also that the voltage supplied to the gate terminal is such that the MOSFET is fully turned on. An attempt to operate below such a minimum value often causes the MOSFET to break because of the driving conditions that cause it to operate in the linear zone rather than in the ohmic zone.
Additionally, it is preferred to delay the start of operation of the integrated control circuit from the moment at which the input voltage is applied to the power supply. Besides realizing a well defined system startup sequence, this is absolutely necessary in some cases.
In network power supplies, during startup, there is a very elevated current pulse absorption due to the fact that such systems have at input a bridge rectifier followed by a filter capacitor that, initially, is uncharged and operates as a short circuit until it has been charged. To avoid the current reaching dangerous values for the bridge diodes and the filter capacitor, some current limitation means are set (for example, a resistance). However, because they are dissipating, they must be disconnected once they are not necessary any more. Therefore, it is advantageous to use a controlled switch placed in parallel with the limitation element, which is initially open and is closed as soon as the power supply starts its operation so as to be short-circuited. It is therefore necessary to introduce a delay at the start of operation of the integrated control circuit so as to assure that it starts when the input capacitor charging transient is ended.
For all these reasons the integrated control circuits are normally provided with the function known as Undervoltage Lockout (UVLO).
A schematic of part of a conventional integrated control circuit of a switching power supply is shown in FIG. 1. The network voltage Vac is applied through the activation of a switch SW to a diode bridge 10 and a filter capacitor Cf. The voltage Vin, which is provided across the terminals of the capacitor Cf, is applied to the startup circuit 11. The startup circuit 11, which is constituted by a resistance in the simplest case, provides a current Is that charges a second capacitor Cs. To the second capacitor Cs there is also applied a voltage coming from a secondary Wa of the power supply transformer, through a resistance R and a diode D. A fraction Iq of the current Is supplies the integrated control circuit 12. It is applied both to the UVLO circuit 13, and to the power supply driving circuit 14 that provides the control voltage Vg to the power switching element. The UVLO circuit 13 includes a comparator 15 with hysteresis that compares the supply voltage Vcc of the UVLO circuit 13 with a starting voltage Vss. The output voltage of the comparator 15 controls a controlled switch SW1 that opens or closes the supply of the driving circuit 14. The voltage Vin is the voltage that is applied to the power switching element of the power supply.
The network voltage Vac is applied to the power supply by closing the switch SW so that the filter capacitor Cf is charged, in a few milliseconds, to the network peak voltage, so as to originate the voltage Vin.
The startup circuit 11 provides a current Is that partly charges the capacitor Cs, while another part Iq is absorbed by the integrated control circuit 12. The absorption Iq of the integrated control circuit 12 under these conditions is very small because the UVLO circuit 13 maintains the switch SW1 open. The current provided by the startup circuit 11 therefore goes, for the greatest part, to charge the capacitor Cs, so as to increase the voltage Vcc provided across its terminals.
The voltage Vcc continues rising until it reaches the starting value Vss, in a variable time usually from some hundreds of milliseconds to some seconds. During this whole time the driving circuit 14 is turned off, and its output voltage Vg, which is used for driving the MOSFET gate, remains zero. As soon as the voltage Vcc reaches the voltage Vss, the comparator 15 closes the switch SW1, so that the current Iq considerably increases; the driving circuit of the MOSFET is enabled and the activity of the power supply begins.
The increased consumption of the device is not sustained by the startup circuit 11 so there is a quick decrease in Vcc. This is the reason that the comparator of the UVLO circuit 13 is provided with hysteresis. To turn off the driving circuit 14 and go back to the conditions before startup, it is necessary that Vcc drops below a second threshold Vstop that is less than Vss. Without this hysteresis a continuous alternation of turning on and turning off would be experienced.
In the meantime, because of the MOSFET switching, the output voltage of the power supply increases quickly together with the voltage, which is proportional to it, generated by the winding Wa, which is coupled to the transformer driven by the MOSFET. The winding Wa, the resistance R, the diode D, and the capacitor Cs form a circuit commonly known as the self supply circuit, to which the assignment of sustaining the operation of the integrated circuit is submitted. The number of turns of the winding Wa are opportunely chosen so that the voltage produced is higher than Vstop, and the capacitor Cs is opportunely chosen so that the voltage produced by the winding Wa becomes higher than the voltage Vstop before the voltage Vcc becomes smaller than the voltage Vstop.
The presence of the threshold voltage Vstop also assures a defined and sure operation during the turning off phase. In fact, by opening the switch SW the power supply is supplied at the expense of the charge present on the capacitor Cf, so that its voltage quickly drops. As soon as this voltage becomes insufficient to maintain the power supply active with the current load, the output voltage and, with it Vcc, quickly decrease and go down below the voltage Vstop. As soon as this happens the driving circuit 14 is turned off, Iq returns to its very low initial value, Vg goes to zero, and the MOSFET turns off.
Ideally, the voltage provided by the winding Wa, present across the terminals of the capacitor Cs, is hooked through the coil ratio of the transformer to the output controlled voltage, and it is therefore maintained controlled by the control system. In actual operation this result is almost true when the power supply input voltage varies, but the situation is very different when the load varies.
This is mainly due to the parasitic parameters of the transformer, because of which at high load the voltage goes up a lot more than expected because of the peaks present on the positive leading edges of the voltage on Wa, while at low or null load, where the peaks are extremely lower and the load on Wa represented by the integrated control circuit 12 can also be higher than at output, the voltage decreases notably below the expected value.
In the most modern integrated control circuits 12, this is emphasized by the adoption of some special techniques aimed at minimizing the power supply consumption at low loads so as to facilitate conformity with the most recent regulations regarding the consumption reduction of equipment under non-operating conditions (for example, EnergyStar, Energy2000, Blue Angel, and the like). Such techniques involve, substantially, the reduction of the power supply operating frequency at low or null load, so that the energy that Wa is able to transfer is decreased.
Another problem is represented by the fact that the voltage Vcc cannot overcome a set value Vccmax for reasons related to the technology of the integrated control circuit 12 that impose some limits on the voltage applied to it and, at the same time, under conditions of low or null load, Vcc has to maintain itself higher than Vstop, or the system will operate intermittently. Therefore, the variations of the voltage produced by Wa have to be contained, with some safety margin, within the interval Vstop−Vccmax.
To subsequently complicate the panorama, the requirement that the voltage produced by Wa, under short circuit conditions at the output of the power supply, should be lower than the voltage Vstop, is also added. So an intermittent operation is obtained that limits the power in use to non-dangerous values for the system integrity. It is understood from the description above that under short circuit conditions the peaks produced on Wa are particularly elevated and can be sufficiently energetic to sustain the voltage Vcc above Vstop, where, ideally, the voltage generated by Wa should be near zero.
To contain the phenomenon of a too high voltage at a maximum load and to assure the intermittent operation under short circuit conditions, and further to optimize the constructive formalities of the transformer, usually the resistance R in series with the diode D is used to round off the peaks. Sometimes, a small coil is alternatively used. Nevertheless, both solutions stress the decreasing of Vcc at low or null load. Also, by optimizing the value of such resistor or coil (that is, using the minimum value) so as to assure operation under safety conditions both at maximum load (Vcc<Vccmax) and at short circuit (Vcc<Vstop), it is hardly possible to satisfy the condition Vcc>Vstop at low or null load. To solve this last problem some ballast loads at the power supply output are then added so as to contrast the decreasing of Vcc. This, however, worsens the system efficiency and, above all, it practically makes it impossible to satisfy the various regulations (such as EnergyStar, Energy2000, Blue Angel, and so on).
The same drawbacks also occur with other external circuital solutions that are used to minimize the peak effect. In all of them, satisfying the conditions Vcc<Vccmax at full load and Vcc<Vstop in short circuit makes it extremely difficult to also satisfy the condition Vcc>Vstop at minimum or null load.